1. Field of the Invention
This invention relates to a moisture meter for measuring the water content in a specimen through measuring the electric resistance of the specimen, and more particularly to such a moisture meter provided with temperature compensation.
Description of the Prior Art
In grains such as rice or wheat, the water content was a correlation with the electric resistivity and hence it can be discovered by measuring the electric resistivity.
The electric resistivity of grain with a constant water content, however, shows a temperature dependence corresponding to the water content of .+-. 1.0% for the temperature change of .+-. 10.degree.C. Thus, temperature compensation should be done in measuring the water content of a grain specimen from the electric resistivity. It has been done by compensating the measured result with a temperature compensation table, or with a compensating quantity which is arranged to be directly read from a thermometer. "Grain moisture meter, model SP-1" (since April, 1967) of Kett Electric Laboratory in Japan and "moisture meter, model TW73" (trade name "Protimeter Grain-master") (Since 1973) of Protimeter Ltd. are examples of these types.
According to such methods, however, temperature compensation is troublesome and time-consuming.
It has been known that at a constant temperature the electric resistivity of grain varies with the water content as is shown in FIG. 1, i.e. it varies exponentially in the water content range of 12 to 20% and relatively linearly in the water content range of 20 30%. However, in performing such a method with one range of reading, there occur problems of making the circuit structure extremely complicated and the adjustment of the circuit extremely difficult, thereby bringing it into practice being difficult. Thus, it can be considered to divide the whole range into a low water content range of 12 to 20% and a high water content range of 20 to 30%.
As an attempt in linearizing the indication scale in a moisture meter, Japanese Patent Publication No. 35039/1973 discloses such an electric resistance measuring apparatus in that an electric signal is amplified in an amplifying transistor in the low water content range in which it was difficult to measure fine differences in the water content because of its high resistivity, an electric signal is by-passed through a compensating transistor used in diode-like manner in the high water content range to decrease the rate of change, and the water content is preliminarily converted into the current flowing through a reference resistance and indicated in a meter to enable direct reading. In this apparatus, however, no consideration is made of the temperature compensation which is indispensable in the water content measurement of grain specimens.